The present invention generally relates to arc control for spraying and more specifically relates to electric arc control in a thermal spraying process wherein an electrode is a consumable spray material.
Plasma metal spraying processes using wire feed stock are generally well known, and are discussed at length in U.S. Pat. No. 5,245,153 entitled Depositing Metal Onto a Surface which issued to Singer et al.; U.S. Pat. No. 5,707,693 entitled Method and Apparatus for Thermal Spraying Cylindrical Bores which issued to Vliet et al.; U.S. Pat. No. 5,796,064 entitled Method and Apparatus for Dual Coat Thermal Spraying Cylindrical Bores which issued to Rice et al.; and U.S. Pat. No. 5,938,944 entitled Plasma Transferred Wire Arc Thermal Spray Apparatus and Method which issued to Baughman et al. All of these patents are herein incorporated by reference. The primary process control objective in a thermal spray process is uniform melt rate of the wire feed stock. The melt rate is a function of: wire feed rate; supply current at the plasma gun; voltage potential between the wire feed stock and the gun electrode; and other factors such as gun design, gas flow, etc., that are generally presumed constant throughout the following disclosure. A secondary concern is wire xe2x80x9cburn backxe2x80x9d that affects the melt rate and can damage the plasma gun due to double arcing that can occur when the consumable electrode (feed wire) burns back too far due to a fault in feeding the wire. With current practices, burn back is generally caused by a slower wire feed rate which may be due to drag on the feed wire and/or slippage of the drive rollers.
Current practices use a constant wire feed rate once the process is tuned along with a constant current supply. This method of controlling the arc in a thermal spray process is taught in U.S. Pat. No. 5,296,667 entitled High Velocity Electric-Arc Spray Apparatus and Method of Forming Materials which issued to Marantz et al. and is herein incorporated by reference. This method, however, requires a more expensive type of power supply.
Marantz et al. also teaches a method for preventing damage to a spray gun due to burn back that operates by sensing a rise in voltage produced by a constant current power supply. Burn back increases the plasma arc length, which, in a system with a constant current supply, results in a higher voltage across the electrodes. A disadvantage of a constant current system is by definition a variable voltage system that increases voltage if current starts to fall or decreases voltage if current starts to rise. As current remains essentially constant, the rise and fall of voltage corresponds to a rise and fall in heat energy in the arc. In the case of burn back, the arc increases in length which corresponds to an increase in resistance (R). To compensate, voltage rises to maintain a constant current level, which results in more burn back. The opposite effect occurs with constant voltage control. The method of Marantz et al. includes interrupting the power supply upon excessive voltage rise before damage to the gun can occur. Thus, a mechanism is used for sensing xe2x80x9cburn backxe2x80x9d that uses a voltage sensing circuit in line with a power supply to the thermal spray gun and turns off power to the system if burn back occurs. Hence, the method does not succeed in reducing occurrence of burn back by compensating for onset of a burn back condition, but merely prevents damage to the spray gun at a cost of shutting down the thermal spraying process an d interrupting production. Also, the method fails to account for variations in power characteristics that may result, for example, from power surges and/or faulty electrical contacts in the system. Further, where a change in thermal energy is detected due to an excessive rise in applied voltage, the method teaches turning off power to the system in response. Hence, the method does not have adaptive capability to maintain even deposition of thermal spray material on a workpiece despite changes in thermal energy, but merely prevents uneven deposition of thermal spraying material on a workpiece in some conditions at a cost of shutting down the thermal spraying process in other conditions.
The need remains, therefore, for an adaptive system and method of arc control in a thermal spraying system that decreases occurrence of burn back conditions while maintaining even deposition of spray material on a workpiece despite variations in power supplied to the thermal spray gun. The present invention supplies such a solution.
In accordance with the present invention, a thermal spraying system includes consumable electrode thermal spray material, an electrical energy thermal spraying device atomizing and spraying the thermal spray material, a power supply supplying electrical power to the spraying device, a sensor monitoring a power characteristic of the supplied electrical power, and a feed rate control module adjusting a feed rate of the thermal spray material based on the monitored power characteristic.
In another aspect, a thermal spraying system includes consumable electrode thermal spray material, an electrical energy thermal spraying device atomizing and spraying the thermal spray material, a workpiece receiving atomized and sprayed thermal spray material on a surface of the workpiece, a power supply supplying electrical power to the spraying device, a sensor monitoring a power characteristic of the supplied electrical power, and a movement rate control module adjusting a rate of movement of the spraying device relative to the workpiece based on the monitored power characteristic.
In another aspect, the present invention is a thermal spraying system including consumable electrode thermal spray material, an electrical energy thermal spraying device atomizing and spraying the thermal spray material, a constant voltage DC power supply operable to supply electrical power to the spraying device, a current sensor operable to monitor an electrical current level of the supplied electrical power, and a power interruption module operable to shut off electrical power to the spraying device if the sensed electrical current level drops below a predetermined threshold in order to reduce damage to the spraying device that occurs in a burn back condition.
In another aspect, the present invention is a thermal spraying system including consumable electrode thermal spray material, an electrical energy thermal spraying device atomizing and spraying the, thermal spray material, a power supply supplying electrical power to the spraying device, a sensor monitoring a power characteristic of the supplied electrical power, and a feed rate control module adjusting a feed rate of the thermal spray material based on the sensed power characteristic in order to reduce damage to the spraying device that occurs in a burn back condition without shutting down said system.
In another aspect, the present invention is an arc control system for use with a spraying device atomizing and spraying consumable electrode thermal spray material. The system includes an input module receptive of power characteristic information relating to a power characteristic of electrical power supplied to the spraying device, and a feed rate control module generating feed rate control information based on the power characteristic information, wherein the feed rate control information is calculated to adjust a feed rate of the thermal spray material.
In another aspect, the present invention is an arc control system for use with a spraying device atomizing and spraying thermal spray material. The system includes an input module receptive of power characteristic information relating to a power characteristic of electrical power supplied to the spraying device, and a movement rate control module generating movement rate control information based on the power characteristic information, wherein the movement rate control information is calculated to adjust a rate of movement of the spraying device relative to a workpiece, thereby improving evenness of deposition of atomized and sprayed thermal spray material on a surface of the workpiece.
In another aspect, the present invention is a method of performing arc control for use with a spraying device atomizing and spraying consumable electrode thermal spray material. The method includes receiving power characteristic information relating to a power characteristic of electrical power supplied to the spraying device, generating feed rate control information based on the power characteristic information, wherein the feed rate control information is calculated to adjust a feed rate of the thermal spray material, and adjusting the feed rate based on the feed rate control information.
In another aspect, the present invention is a method of controlling a spraying device atomizing and spraying thermal spray material. The method includes receiving power characteristic information relating to a power characteristic of electrical power supplied to the spraying device, generating movement rate control information based on the power characteristic information, wherein the movement rate control information is calculated to adjust a rate of movement of the spraying device relative to a workpiece in order to improve evenness of deposition of atomized and sprayed thermal spray material on a surface of the workpiece, and adjusting the rate of movement of the spraying device relative to said workpiece based on the rate control information.
The arc control system of the present invention is advantageous over conventional devices in that it can control the current at a constant level without requiring a constant current DC power supply. This capability is particularly advantageous because a constant current DC power supply is considerably more expensive than a substantially constant voltage DC power supply. Especially for multiple thermal spray gun applications where the use of a constant current DC power supply necessitates a separate power supply to each thermal spray gun, a substantially constant voltage power supply allows use of only one power supply to all thermal spray guns. In many applications, plant line voltage regulation is satisfactory for voltage control. An additional advantage of the arc control system of the present invention is an ability to include a mechanism that prevents damage to a thermal spray gun due to a burn back condition without requiring a constant current DC power supply. A further advantage of the present invention is that it can sense the onset of a burn back condition that could lead to damage of the thermal spray gun, and act to prevent the burn back condition without shutting down the thermal spray process by increasing the feed rate of the consumable electrode spray material. A still further advantage of the arc control system of the present invention is that it can monitor thermal energy as a function of supplied power characteristics and seek to maintain a balance between thermal energy and feed rate by varying feed rate of the consumable electrode spray material. An even further advantage of the present invention is that it can vary a traverse rate of the thermal spray gun relative to a workpiece in response to a monitored change in thermal energy in as correlated with fluctuations in supply voltage and/or current. Thus, the present invention can improve uniformity of deposition of thermal spray material on a workpiece. Additional advantages and features of the present invention will become apparent from the following detailed description, claims, and accompanying drawings.